One hundred millimeter single crystal silicon carbide wafer

ABSTRACT

A method of producing a high quality bulk single crystal of silicon carbide in a seeded growth system is disclosed. The method includes positioning the seed crystal in a crucible while exerting minimal torsional forces on the seed crystal to thereby prevent torsional forces from warping or bowing the seed crystal in a manner that that would otherwise encourage sublimation from the rear of the seed crystal or undesired thermal differences across the seed crystal.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a continuation of Ser. No. 10/876,963 filed Jun. 25, 2004 forOne Hundred Millimeter High Purity Semi-Insulating Single CrystalSilicon Carbide Wafer (now published as No. 20050126471). Ser. No.10/876,963 is a continuation in part of copending and commonly assignedapplication Ser. No. 10/064,232 filed Jun. 24, 2002 for “Method forProducing Semi-Insulating Resistivity in High Purity Silicon CarbideCrystals” (now published as No. 20030233975). Ser. No. 10/876,963 alsoclaims priority from copending application Ser. Nos. 10/628,188 filedJul. 28, 2003 for “Reducing Nitrogen Content in Silicon Carbide Crystalsby Sublimation Growth in a Hydrogen-Containing Ambient” (now Publishedas No. 20050022727); 10/628,189 filed Jul. 28, 2003 for “Growth ofUltra-High Purity Silicon Carbide Crystals in an Ambient ContainingHydrogen” (now published as No. 20050145164); and 10/814,075, filed Mar.31, 2004 for “Process for Producing High Quality Large Size SiliconCarbide Crystals” (now abandoned).

The present invention also relates to the inventions set forth incommonly assigned U.S. Pat. Nos. 6,218,680; 6,396,080; 6,403,982;6,639,247; and 6,507,046. These issued patents are incorporated entirelyherein by reference.

STATEMENT OF GOVERNMENT INTEREST

The U.S. Government has a paid-up license in this invention and theright in limited circumstances to require the patent owner to licenseothers on reasonable terms as provided for by the terms of Contract No.N00014-02-C-0306 awarded by Defense Advanced Research Projects Agency(DARPA).

BACKGROUND

The present invention relates to semi-insulating silicon carbide singlecrystals, and in particular, relates to methods of forming high puritysemi-insulating silicon carbide single crystal wafers that are at least100 millimeters (mm) in diameter. Because of the relationship betweenEnglish and Metric units (e.g., 25.4 mm=1 inch), such wafers are alsoreferred to as “four inch” wafers.

Silicon carbide (SiC) has a combination of electrical and physicalproperties that make it an attractive semiconductor material for hightemperature, high voltage, high frequency and high power electronicdevices. These properties include a 3.0 electron-volt (eV) bandgap (6H),a 4 Megavolt per centimeter (MV/cm) electric field breakdown, a 4.9W/cmK thermal conductivity, and a 2×10 7 centimeter per second (cm/s)electron drift velocity. Silicon carbide is also particularly useful inits ability to be made conductive by doping or semi-insulating byvarious processing techniques. These qualities make silicon carbide amaterial of choice for a vast array of electronic applications.

The production of integrated circuits for many applications, such as RFdevices, requires a semi-insulating substrate on which electronicdevices can be built and connected to one another. Historically,sapphire was used as substrate material for microwave devices because ofits high resistance to current flow. Sapphire has the disadvantage,however, of limiting the types of semiconductor layers that may befabricated on the substrate with appropriate crystal lattice matchingfor proper device operation.

As used herein, the terms “high resistivity” and “semi-insulating” canbe considered synonymous for most purposes. In general, both termsdescribe a semiconductor material having a resistivity greater thanabout 1500 ohm-centimeters (ohm-cm). In this regard, semi-insulatingsilicon carbide devices should have a substrate resistivity of at least1500 ohm-centimeters (ohm-cm) in order to achieve RF passive behavior.Furthermore, resistivities of 5000 ohm-cm or better are needed tominimize device transmission line losses to an acceptable level of 0.1dB/cm or less. For device isolation and to minimize back-gating effects,the resistivity of semi-insulating silicon carbide should approach arange of 50,000 ohm-cm or higher.

Research in the field shows that the semi-insulating behavior of asilicon carbide substrate is the result of energy levels deep within theband gap of the silicon carbide; i.e., farther from both the valenceband and the conduction band than the energy levels created by p-typeand n-type dopants. These “deep” energy levels are believed to consistof states lying at least 300 meV away from the conduction or valenceband edges, e.g., U.S. Pat. No. 5,611,955 which is representative ofstandard prior research in this art.

As set forth in the above-referenced patents and applications, it hasbeen discovered that semi-insulating silicon carbide can be producedwithout the use of vanadium as the dopant to create deep level statesthat produce the semi-insulating character.

Although vanadium can produce a semi-insulating silicon carbide crystal,its presence has been observed to create a back-gating effect; i.e., thetrapped negative charge on the vanadium acts as a grown-in gate indevices in which a vanadium-doped crystal is used as the semi-insulatingsubstrate. Thus, for certain device considerations, vanadium is bestavoided.

In the above-referenced issued patents, a semi-insulating siliconcarbide crystal is described that includes donor dopants, acceptordopants and intrinsic point defects that produce deep level states. Whenthe concentration of intrinsic point defects exceeds the differencebetween the concentration of donors and the concentration of acceptors,the states resulting from intrinsic point defects can providesemi-insulating characteristics in the functional absence of vanadium;i.e., including a minimal presence that is less than the presence thatcan affect the electronic properties of the crystal.

The requirements for and the advantages of semi-insulating substrates,their use in devices, particularly microwave devices, and the associatedand particular requirements for silicon carbide semi-insulatingsubstrates are set forth in detail in the above-referenced issuedpatents, and are generally well understood in the art from a backgroundstandpoint. Thus, they will not be repeated in detail herein. Forreference purposes, a relevant discussion is set forth in No. 6,218,680(‘the '680 patent”) at column 1, line 14 through column 3, line 33.

To this discussion it should be added, however, that the ever-increasingdemand for wireless communication services, including high bandwidthdelivery of Internet access and related services, drives a correspondingdemand for devices and circuits that can support such delivery, which inturn calls for materials—such as semi-insulating silicon carbide—fromwhich devices having the required capabilities can be manufactured.

Accordingly, the '680 patent explains that superior microwaveperformance can be achieved by the fabrication of silicon carbide fieldeffect transistors (FETs) and related devices on high purity,vanadium-free semi-insulating monocrystalline silicon carbidesubstrates. As set forth in the '680 patent, the substrates derive theirsemi-insulating properties from the presence of intrinsic (point defectrelated) deep electronic states lying near the middle of the siliconcarbide bandgap. The intrinsic deep states generally arise during growthof a crystal boule at high temperatures from which substrate wafers arecut in a manner generally well understood in this art.

In devices that incorporate these substrates, and in order to providethe appropriate low-loss RF performance, the substrate must act as alow-loss dielectric medium by continuously maintaining itssemi-insulating characteristics. In turn, the ability to maintainsemi-insulating behavior is dependent upon the total number of intrinsicdeep states in the substrate. In current practice, if the density of theintrinsic deep levels is not sufficiently high it has been observed inpractice that the semi-insulating characteristics of the substrate canbecome reduced or functionally eliminated when subsequent steps arecarried out on or using a semi-insulating silicon carbide wafer. Suchsteps include the growth of epitaxial layers at temperatures of about(for illustrative purposes) 1400° or above on the semi-insulatingsilicon carbide wafer. This in turn reduces the number of useful devicesthat can be formed on or incorporating the wafers.

Although the inventors do not wish to be bound by any particular theory,it appears that when semi-insulating silicon carbide substrate wafers ofthis type are subjected to process steps at temperatures within certainranges, the subsequent processing can act as an anneal that reduces thenumber of point defects. This can be thought of in the positive sensethat a higher quality crystal is created, but it is disadvantageous whenthe number of intrinsic point defects is the basis for thesemi-insulating character of the substrate wafer.

Stated differently, if kept within a particular temperature range for asufficient time, the crystal equilibrium or near-equilibrium will shiftto one in which the number of point defects is reduced; i.e., thecrystal becomes more ordered (fewer point defects) at lower temperaturesthan it was at higher temperatures, in a manner expected in accordancewith well-understood thermodynamic principles.

As another factor, silicon carbide grown by most techniques is generallytoo conductive for semi-insulating purposes, however. In particular, thenominal or unintentional nitrogen concentration in silicon carbide tendsto be high enough in sublimation grown crystals (≧1−2×10¹⁷ 17/cm³) toprovide sufficient conductivity to prevent silicon carbide from beingused in devices that require a semi-insulating substrate, such asmicrowave devices.

A recurring issue in fabricating silicon carbide crystals for electronicdevices, therefore, is the control of elemental impurities such asnitrogen within the crystal. Nitrogen content, for example, affects thecolor of a silicon carbide crystal. This color change can havedeleterious consequences for the usefulness of a crystal in certainapplications requiring luminescence, such as light emitting diodes andgemstone fabrication. The nitrogen in a crystal may also yieldelectrical conductivity that must be controlled for silicon carbide tohave appropriate properties in diverse electronic applications. Theinvention herein includes a means for achieving a semi-insulatingsilicon carbide crystal one step of which comprises reducing thenitrogen content, and therefore the inherent conductivity of a crystalwith an improved method of sublimation growth in a hydrogen ambientatmosphere.

Researchers, therefore, persistently struggle with the issue ofcontrolling, and particularly reducing, the amount of nitrogen that istransferred from the atmosphere of a sublimation growth chamber into agrowing silicon carbide crystal. Commonly assigned U.S. Pat. No.5,718,760 to Carter et al., for example, discloses a method of reducingthe nitrogen concentration in the ambient atmosphere of a siliconcarbide sublimation system. The Carter '760 patent reduces the nitrogenby back filling the growth chamber with an inert gas such as argon andthen evacuating the growth chamber to a very low pressure.

Another technique for decreasing the ambient nitrogen in a crystalgrowth system is the minimization of nitrogen content in the equipmentitself. Commonly assigned U.S. Pat. No. 5,119,540 issued to Kong et al.,discloses that most, if not all, of the undesired nitrogen in a crystalgrowth system is a result of nitrogen gas that escapes from theequipment itself. For example, nitrogen trapped in graphite equipmentmay leak into the ambient atmosphere because the equipment cracks ordevelops pin holes through which nitrogen escapes at very hightemperatures. The Kong '540 patent prevents incorporation of nitrogeninto subject silicon carbide crystals by utilizing fabrication equipmentmade of materials with low nitrogen concentration. The Kong '540 patent,therefore, teaches that extremely pure equipment components that arefree of high nitrogen content result in silicon carbide crystals thatare less contaminated with undesirable levels of nitrogen. Kong '540shows nitrogen minimization in a chemical vapor deposition system but isequally pertinent in the sublimation systems discussed herein.

In addition to reducing the concentration of nitrogen, researchers alsoreduce the effects of unavoidable nitrogen content within a siliconcarbide crystal. For example, the Carter '760 patent acknowledges thatthe background nitrogen in the sublimation chamber can lead toundesirable crystal color. The '760 patent, therefore, discloses amethod of compensating the nitrogen content with a corresponding p-typedopant to minimize or eliminate the undesirable effects of the nitrogen.The p-type dopant and the nitrogen compensate one another and preventundesirable color centers in the preferably colorless silicon carbidecrystal of the Carter '760 invention.

The nitrogen compensation technique has also been used to preventunintentional nitrogen doping from dominating the conductivity ofsilicon carbide crystals. The '680 patent discloses a further method ofcompensating the nitrogen content of a silicon carbide crystal grown bysublimation. Carter points out that boron may be used to compensate theinherent nitrogen. Carter '680 also utilizes the temperature gradient inthe disclosed sublimation process to create point defects in a siliconcarbide crystal. The Carter '680 technique pairs an undesirable nitrogenconcentration in the silicon carbide crystal with a correspondingacceptor dopant, such as boron. Carter '680 then pairs any excessdopants with temperature induced point defects to yield a desiredsemi-insulating crystal.

Other research also concedes that unintentional nitrogen incorporationoccurs in silicon carbide crystals grown by sublimation. This researchtends to focus on means for minimizing the effects of the undesirablenitrogen concentration instead of preventing the nitrogen incorporationfrom the outset. The '955 patent cited above and issued to Barrett etal. is illustrative of this point. Barrett '955 shows a means ofintroducing elements such as vanadium into the semiconductor materialthat create deep energy states within the forbidden energy gap. TheBarrett '955 method accounts for nitrogen content in a silicon carbidecrystal by trapping the nitrogen and hindering electron mobility fromthe nitrogen. Barrett, therefore, achieves a semi-insulating siliconcarbide substrate by adjusting the effects of the nitrogen instead ofpreventing its presence in the crystal.

The techniques set forth in the two Carter patents, which have a commonassignee as the invention described and claimed herein, are useful fortheir respective purposes to minimize the effects of nitrogenincorporation in a silicon carbide crystal. The Barrett '955 patentrequires further elemental doping and can give rise to unpredictableelectrical responses in a subject silicon carbide crystal.

A need continues to exist, therefore, for a method of gaining extensivecontrol over the incorporation of nitrogen into a silicon carbidecrystal at the point of initial sublimation. By controlling the nitrogencontent from the initial growth of the crystal, compensation techniquesand the associated process steps may be minimized. Controlling thenitrogen incorporation also allows development of more diverse types ofcrystals, including crystals with varying degrees of nitrogen contentfor specialized purposes.

Silicon carbide is, however, a difficult material to work with becauseit can crystallize in over 150 polytypes, some of which are separatedfrom one another by very small thermodynamic differences. Furthermore,because of silicon carbide's high melting point (over 2700° C. underhigh pressure), many processes for working silicon carbide, includingepitaxial film deposition, often need to be carried out at much highertemperature than analogous reactions in other semiconductor materials.

In one sense the commercial or manufactured synthesis of siliconcarbide—typically for use as an abrasive—has been carried out for wellover a century, with growth of polycrystalline silicon carbide beingrecognized by Acheson in 1885. The genesis of growth for electronicpurposes, however, was initiated much later, being generally attributedto the development of the “Lely” method (named after its inventor) in1955. In 1978, the first seeded sublimation techniques, which are alsoreferred to as modified Lely techniques, were carried out, and by thelate 1980s, silicon carbide technology was being rapidly commercialized,primarily by the assignee of the present invention.

In a seeded sublimation technique, a seed crystal and a source powderare both placed in a reaction crucible which is heated to thesublimation temperature of the source and in a manner that produces athermal gradient between the source and the marginally cooler seedcrystal. The thermal gradient encourages vapor phase movement ofmaterials from the source to the seed followed by condensation upon theseed and the resulting bulk crystal growth. The method is also referredto physical vapor transport (PVT).

In a typical silicon carbide growth technique, the crucible is made ofgraphite and is heated by induction or resistance, with the relevantcoils and insulation being placed to establish and control the desiredthermal gradients. The source powder is silicon carbide, as is the seed.The crucible is oriented vertically, with the source powder in the lowerportions and the seed positioned at the top, typically on a seed holder;see U.S. Pat. No. 4,866,005 (reissued as No. RE34,861). These sourcesare exemplary, rather than limiting, descriptions of modern seededsublimation growth techniques.

One typical and useful method for attaining high quality crystal growthis the epitaxial growth method in which a thin film of a semiconductormaterial is deposited upon a substrate of the same or acrystallographically similar material. In many circumstances, epitaxialgrowth is carried using chemical vapor deposition (“CVD”). This refersto the technique of introducing flowing source gases that contain therelevant elements into a reaction chamber in the presence of a suitablesubstrate under conditions of temperature, pressure and flow rate thatencourage the source gases to react and form a high quality crystal ofthe desired material on the substrate.

For a number of reasons, the highest quality epitaxial layers (or“epilayers”) of any given semiconductor material are generally bestformed upon a substrate of the same material; e.g., silicon carbideepilayers on silicon carbide substrates. Doing so helps eliminatecrystal lattice mismatches or other physical incongruence such asdifferent coefficients of thermal expansion that are almost alwaysnecessarily present when two different, even if quite similar, materialsare in contact with one another.

Accordingly, attaining suitable substrate materials for both epitaxialgrowth and for devices is a fundamental requirement in semiconductordevice manufacture. As another factor, although epitaxial growth tendsto produce high quality crystals, it proceeds very slowly. As a result,using epitaxial growth techniques to obtain bulk single crystals tendsto be commercially unattractive even if physically possible.

Therefore, increasing the rate at which large single crystals of siliconcarbide can be grown, increasing the diameter to which they can begrown, and reducing the defect density in the large crystals remainnecessary and desired goals.

Although the density of structural defects in silicon carbide bulkcrystals has been continually reduced in recent years, relatively highdefect concentrations still appear and have been found to be difficultto eliminate. These can cause significant problems in limiting theperformance characteristics of devices made on the substrates, or insome cases can preclude useful devices altogether. For example, atypical defect density in some commercially available silicon carbidewafers can be on the order of 100 per square centimeter (cm-2). Amegawatt device formed in silicon carbide, however, will require adefect-free area on the order of 0.4 cm-2. Thus, obtaining large singlecrystals that can be used to fabricate large surface area devices forhigh-voltage, high current applications remains difficult.

Although occasionally named differently, the most common defects insilicon carbide bulk crystals are generally referred to as micropipesand hexagonal voids. A micropipe is a hollow core super-screwdislocation with its Burgers vector lying along the c-axis. A number ofcauses have been proposed or identified for the generation ofmicropipes. These include excess materials such as silicon or carboninclusions, extrinsic impurities such as metal deposits, boundarydefects, and the movement or slippage of partial dislocations. See e.g.,Powell et al., Growth of Low Micropipe Density SiC Wafers, MaterialsScience Forum, Vols. 338-340, pp 437-440 (2000).

Hexagonal voids are flat, hexagonal platelet-shaped cavities in thecrystal that often have hollow tubes trailing beneath them. Someevidence shows that micropipes are associated with hexagonal voids. Arelatively recent discussion of such defects (exemplary and notlimiting) is set forth in Kuhr et al., Hexagonal Voids and the Formationof Micropipes During SiC Sublimation Growth, Journal of Applied Physics,Volume 89, No. 8, page 4625 (April 2001).

Recent research indicates that problems in the bulk crystals produced ina seeded sublimation technique can originate with the seed itself andthe manner in which it is physically handled; e.g., Sanchez et al.,Formation of Thermal Decomposition Cavities in Physical Vapor Transportof Silicon Carbide, Journal of Electronic Materials, Volume 29, No. 3,page 347 (2000). Sanchez uses the term “micropipe” to describe,“approximately cylindrical voids with diameters in the range of 0.1 μmto 5 μm that form at the core of superscrew dislocations alignedparallel or nearly parallel to the [0001] axis” Id. at 347. Sanchezrefers to larger voids (“diameters from 5 μm to 100 μm”) as, “thermaldecomposition cavities,” and opines that micropipes and thermaldecomposition cavities arise from different causes. Id.

Based on this hypothesis and his experimental work, Sanchez suggeststhat migration of “silicon rich vapor” from the back (opposite togrowth) surface of a seed crystal into the growth system, causes thermaldecomposition cavities that generate micropipes in the seed and then inthe growing crystal. Sanchez suggests, “a continuous diffusion barrierfor silicon bearing species,” will reduce or eliminate such cavities.Sanchez describes a carbonized sucrose barrier for this purpose, butadmits that the technique is, “not entirely reproducible.” Id. at 352.

Accordingly, in order to continue to provide improvements in the qualityof single crystal silicon carbide bulk crystals, and to reduce thedefect density, the source of defects at the seed must be identified andsuccessfully addressed.

SUMMARY

The invention is a single polytype single crystal silicon carbide waferhaving a diameter greater than three inches and less than five inches,resistivity greater than 10,000 ohm-cm, a micropipe density less than200 cm⁻² and a combined concentration of shallow level dopants less than5E16 cm⁻³.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating the temperature rangesreferred to in the detailed description and several different coolingrates; and

FIG. 2 is a plot of the change in capacitance against temperature indegrees Kelvin as measured by deep level transient spectroscopy (DLTS).

FIG. 3 is a comparative set of three plots from electron paramagneticresonance (EPR) evaluation of silicon carbide crystal samples.

FIG. 4 shows the low temperature photoluminescence spectrumcorresponding to a 4H-silicon carbide crystal grown in a conventionalargon ambient of the prior art.

FIG. 5 shows the low temperature photoluminescence spectrumcorresponding to a 4H-silicon carbide crystal grown in a hydrogenambient according to the invention herein.

FIGS. 6 and 7 are micrographs reproduced from scientific papers in thisfield, both of which illustrate micropipes in silicon carbide.

FIG. 8 is a cross-sectional schematic view of a conventional sublimationgrowth system for silicon carbide.

FIGS. 9 and 10 are respective cross-sectional schematic diagrams of seedholders, seed crystals, and growing crystals relating to the presentinvention.

FIG. 11 is a cross-sectional schematic diagram of a seed holder andsilicon carbide seed crystal according to the present invention.

FIG. 12 is a schematic illustration of some of the factors related tothe present invention.

DETAILED DESCRIPTION

The present invention is a silicon carbide wafer comprising a singlepolytype single crystal, a diameter greater than 3 inches and less than5 inches, a resistivity greater than 10,000 ohm centimeters (Ω-cm), amicropipe density less than 200 per square centimeter (cm⁻²), and acombined concentration of shallow level dopants less than 5E16 (5×10¹⁶)per cubic centimeter (cm⁻³).

As well-understood by those of ordinary skill in this art, the term“single crystal” (or its equivalent, “monocrystalline”), particularlyfor crystals of the size described and claimed herein, is properlyapplied to such large crystals even when they contain small areas,regions or amounts of other polytypes of silicon carbide. Thus, largehigh-quality crystal wafers, in which a single polytype overwhelminglypredominates, are properly referred to herein as “single crystals.”

In preferred embodiments, the wafer has a diameter of about 100 mm or 4inches. As noted elsewhere herein, the relationship between metric unitsand English units is such that a 4 in. wafer will be 101.6 mm indiameter. Those of ordinary skill in this art recognize these factors,and will understand that the invention covers wafers that are exactly100 mm in diameter, as well as those exactly 4 in. in diameter.

The wafers according to the present invention are suitable for highfrequency power devices, high power devices, high temperature devices,optoelectronic devices, and Group III nitride deposition.

The wafers can be selected from the 3C, 4H, 6H and 15R polytypes ofsilicon carbide with the 4H polytype being presently most preferred. Thewafers are available in an on-axis orientation or various off-axisorientations such as (but not limited to) 3.5°, 4°, and 8°. As oneexample, off-axis or off-orientation wafers can (but do not necessarily)refer to the angle between the projection of the normal vector to thesurface onto a {0001} plane in the projection on that plane of thenearest <11-20> direction.

Wafers according to the invention have less than 200 micropipes persquare centimeter (cm⁻²), more preferably less than 100 micropipes persquare centimeter, in more preferred embodiments less than 30 micropipesper square centimeter, and in low and ultra-low density, less than 15and less than five micropipes per square centimeter respectively.

The wafers can include a single side polish with either the silicon orthe carbon face ready for epitaxial growth or can be double sidepolished with either the silicon or the carbon face ready for theepitaxial growth. The wafers can be finished with a chemical mechanicalpolish with the silicon face epitaxial ready, or with a double sidechemical mechanical polish with the silicon face epitaxial ready.

As used herein, the wafer's diameter refers to the linear dimensionacross the surface of away for width measurement performed manually withANSI certified digital calipers on each individual wafer.

The thickness is measured at the center point with ANSI certifiednon-contact tools. The flatness is similarly measured with certifieddigital calipers on a sample of one wafer per ingot. For 100 mm wafers,the tolerance is within 0.015 in. (0.38 mm). Wafers according to thepresent invention have standard thicknesses of between about 350 and 700microns (μm) depending on intended end use.

The off-axis surface orientation refers to the orientation of thesurface of the wafer with respect to a crystallographic plane within thelattice structure. Surface orientation is measured with an x-raygoniometer on a sample of one wafer per ingot in the center of thewafer.

For commercial purposes, the wafers are available with both primary andsecondary flats. As is conventional in semiconductor terminology, theprimary flat represents the flat of the longest length on the wafer,oriented such that the chord is parallel with a specified low indexcrystal plane. The primary flat is measured on one wafer per ingot usingreflection techniques with manual angle measurements.

The secondary flat is the flat of shorter length than the primary flatand the position of the secondary flat with respect to the primary flatidentifies the face of the wafer. In preferred embodiments, the carbonface of each individual wafer is laser marked with optical characterrecognition compatible fonts consistent with definitions andcharacteristics in the semiconductor industry.

Wafers according to the present invention can also comprise between oneand five epitaxial layers on the surface and these layers are typicallyand preferably selected from the group consisting of silicon carbide andthe Group III nitrides. The electronic properties and advantages of eachof these types of materials are well recognized and well understood inthis art, and thus will not be discussed further in detail herein.

The invention can accordingly include a device that incorporates thewafer and epitaxial layer as just described, and in most circumstances,a relatively large plurality of such devices will be formed on a waferand potentially tested on the wafer before being separated intoindividual devices for end use.

The semi insulating silicon carbide wafer of the invention can providean appropriate substrate for devices (depending on their particulardesign) such as FET's, MOSFET's, JFET's, MESFET's, HFET's, HEMT's,extended drain MOSFET's, and lateral DMOS transistors.

In its semi insulating characteristics, the wafer has a concentration ofnitrogen atoms less than about 2×10¹⁵ cm⁻³ and a concentration ofvanadium less than 1×10¹⁴ cm⁻³.

In preferred embodiments, the wafer has a resistivity of at least about50,000 Ω-cm at room temperature.

As set forth in other parts of the discussion, the presence of pointdefects can affect the semi insulating characteristics, and thus in someembodiments, the wafer has a concentration of point defects that doesnot exceed 5×10¹⁷ cm⁻³.

In other embodiments, the wafer can comprise donor dopants, acceptordopants, and intrinsic point defects. In one embodiment, the number ofdopants of a first conductivity type is greater than the number ofdopants of the other (second) conductivity type and the number ofintrinsic point defects that act to compensate the predominating firsttype dopants is greater than the numerical difference by which the firsttype of doping predominates over the second type of doping, and with theconcentration of transition elements the less than 1×10¹⁶ cm⁻³.

In preferred aspects of this embodiment, the first type dopants aredonors, the second type dopants are acceptors, and the intrinsic pointdefects act as acceptors. Most preferably, the acceptors include boron.As in other embodiments, the concentration of vanadium in the wafer ispreferably less than 1×10¹⁴ cm⁻³.

In another embodiment, the wafer of the invention includes nitrogen andat least one acceptor element having an electronic energy level ofbetween 0.3 and 1.4 electron volts relative to the valence band ofmono-crystalline silicon carbide, with the one acceptor element beingpresent in an amount that over-compensates the nitrogen and pins theFermi level of the silicon carbide substrate to the electronic energylevel of the at least one acceptor element.

In another embodiment, the wafer includes an amount of electricallyactive nitrogen, an amount of electrically active point defects that actas acceptors, and an amount of at least one acceptor element having anelectronic energy level of between 0.3 and 1.4 electron volts relativeto the valence band of mono-crystalline silicon carbide, with thecombined amount of the acceptor element and the point defects beinggreater than the amount of electrically active nitrogen and thus pin theFermi level of the silicon carbide single crystal to the electronicenergy level of the at least one acceptor element.

In another embodiment, the wafer according to the invention includesnon-intentionally introduced nitrogen, scandium, and boron, and with theconcentration of nitrogen being greater than the concentration ofscandium and the concentration of boron being sufficient for the sumconcentration of boron and scandium to overcompensate the nitrogen andpin the Fermi level of the silicon carbide to the level of the scandium.

A number of techniques enhance the production of wafers according to thepresent invention.

First, although the inventors do not wish to be bound by any particulartheory, in one aspect an appropriate method can be best understood in athermodynamic sense. As noted above, one object of the invention is toavoid the use of vanadium to produce semi-insulating character insilicon carbide. Instead, the present invention creates a sufficientlylarge concentration of point defect states in the silicon carbide sothat the concentration remaining after normal semiconductor processingand device manufacture still exceeds the number necessary to producesemi-insulating character.

Those familiar with the nature of silicon carbide and the basis forsemi-insulating characteristics will recognize that there is no specificnumber or concentration of point defects that meets this requirement.Instead, the goal is to minimize the concentration of other dopants(including point defects) in the lattice that could contribute toconductive characteristics, and then exceed that concentration with theconcentration of items, in this case point defects and the resultingstates they create, that create the desired semi-insulatingcharacteristics.

Stated differently, in a compensated crystal the concentration of pointdefects that produce the desired deep level states and the resultingsemi-insulating characteristics must be greater than the netconcentration of the shallow compensating dopants. Thus, asemi-insulating compensated crystal of silicon carbide can have arelatively high concentration of both acceptor and donor atoms, providedthat the number of point defects is in excess of the difference betweenthose concentrations. This concentration of point defects can also beexpressed as the concentration needed to exceed any uncompensatedshallow dopants. It will be understood, of course, that the deep“levels” per se are not the compensating factors; i.e., deep donors donot compensate shallow donors. Instead, those of ordinary skill in thisart recognize that deep acceptors compensate shallow donors and viceversa.

It has been generally found to be more efficient, however, to minimizethe number of potentially compensating donor and acceptor atoms and thusminimize the number of point defects required to exceed the relevantnumerical difference. For example (and for discussion purposes only), ifthe concentration of donor atoms were 2E¹⁷ (2×10¹⁷ cm⁻³) and acceptoratoms 3E17 (3×10¹⁷ cm⁻³), the concentration of point defects would haveto be greater than 1 E17 (i.e. 3E17 minus 2E17). Thus, minimizing thenumber (concentration) of donor and acceptor atoms is the preferred,although not necessary, manner of carrying out the present invention,because it reduces the number of states that need to be created toproduce the semi-insulating character in the crystal.

In another aspect, the invention is a method of producing high qualitysemi-insulating silicon carbide crystals in the absence of relevantamounts of deep level trapping elements. In this aspect the inventioncomprises heating a silicon carbide crystal to a temperature above thetemperature required for chemical vapor deposition (CVD) growth ofsilicon carbide from source gases, but less than the temperature atwhich disadvantageously high rates of silicon carbide sublimation occurunder the ambient conditions to thereby thermodynamically increase theconcentration of point defects and resulting states in the crystal.

The starting crystals are preferably of high purity and are produced bya seeded sublimation technique such as is set forth in U.S. PatentRE34,861 (reissued from No. 4,866,005) or as discussed in Mueller,Status of SiC Bulk Growth from an Industrial Point of View, J. Crystal.Growth v. 211 No. 1 (2000) pp 325-332.

The method then comprises cooling the heated crystal to approach roomtemperature at a sufficiently rapid rate to maintain a concentration ofpoint defects in the cooled crystal that remains greater than the firstconcentration.

Stated somewhat differently, the method comprises the step of coolingthe heated crystal to approach room temperature at a sufficiently rapidrate to reduce the time spent in the temperature range in which thedefects—including, but not limited to, those created by the heatingstep—are sufficiently mobile to be re-annealed into the crystal tothereby produce a silicon carbide crystal with a concentration of pointdefect-related deep level states that is greater than the concentrationof such states in an otherwise identically grown silicon carbide crystalthat has not been heated and cooled in this manner.

The silicon carbide can be heated in the form of a single crystalsilicon carbide wafer or a single crystal silicon carbide boule (with aboule typically being defined as a single crystal structure). In themost preferred embodiments, the crystal has a polytype selected from the3C, 4H, 6H and 15R polytypes of silicon carbide. In the more preferredembodiments, the method comprises heating and cooling a compensatedsilicon carbide crystal, and in most preferred embodiments comprisesheating and cooling a compensated crystal in which the most concentrateddopant is present in an amount of about 5E16 or less. As set forthpreviously, when a compensated crystal is used, the method comprisesheating and cooling a crystal to increase the number of point defects toan amount greater than the concentration of uncompensated shallowdopants. Under normal circumstances, the cooling step comprises coolingthe heated crystal to room temperature. Thus, in another aspect, theinvention is the semi-insulating silicon carbide crystal made by themethod of the invention and its various embodiments.

In the preferred embodiments, the step of heating the silicon carbidecrystal comprises heating the crystal to a temperature of at least about2,000° C. Although 2000° C. is not an exact or necessary lower limit,the generation of point defects is assumed to be thermodynamicallyactivated and thus probably includes an exponential relationship betweentemperature and the concentration of these defects. It has been observedthat a preferred and useful number of point defects are generated attemperatures of 2,000° C. or higher.

In preferred embodiments, the crystal is heated to a temperature ofbetween about 2,000° C. and 2,400° C. at atmospheric pressure. Thesetemperatures provide a useful range at atmospheric pressure. Attemperatures higher than 2400° C. the silicon carbide tends to sublimeat undeniably high rates and thus temperatures above 2400° C. are lessfavored or disadvantageous at atmospheric pressure.

It will be understood by those familiar with the physical properties ofsilicon carbide that sublimation can occur over a relatively wide rangeof high temperatures. At lower portions of this range, the rate ofsublimation is small enough to be of little or no concern. At higherportions of this range, however, the rate of sublimation will be highenough to be disadvantageous. Accordingly, the upper temperature limitof the method of the invention will to some extent be bounded by thesubjective degree of sublimation found to be troublesome in particularcircumstances. As noted above, at atmospheric pressure, 2400° C. hasbeen found to be a convenient upper limit, but is not an absolute one.

The purpose of raising the temperature to 2000° C. or greater is athermodynamic one: in a normally expected fashion, the entropy of thecrystal is higher at higher temperatures, and thus, more of the pointdefects and resulting states that can produce semi-insulating characterare present at higher temperatures. Additionally, those familiar withsilicon carbide and the thermodynamics of crystals will recognize thatas the temperature increases, additional types of states can exist thatdo not occur at lower temperatures. If the heated crystal is properlycooled in accordance with the present invention, these additional typesof states can be preserved and will contribute to the desiredsemi-insulating properties.

Accordingly, heating the crystals to these temperatures creates a moredisordered crystal, and the invention freezes (in a relative sense)these desired states in the crystals as the crystal is returned to roomtemperature. The cooling step is significant because if the crystal isallowed to spend too much time in intermediate temperature ranges, suchas those above about 1400° C., the crystal will undergo theaforementioned annealing process, and can reach a different equilibriumor near-equilibrium condition in which the states disappear (or arefunctionally reduced to an irrelevant number) as the crystal becomesmore ordered.

With respect to the preferred upper limit of 2400° C., it will beunderstood by those familiar with silicon carbide and crystal growthtechniques that this is a practical limitation at atmospheric pressurerather than an absolute one. Stated differently, 2400° C. is a preferredupper temperature when using relatively typical equipment operating atatmospheric pressure. Those having the ordinary skill expected in thisfield could carry out the heating at higher temperatures without undueexperimentation, but would have to add additional equipment andtechniques such as incorporating an overlying silicon and carbonatmosphere or using some other high pressure technique to prevent thesublimation of silicon carbide that begins to occur in statisticallysignificant amounts at such higher temperatures.

Thus, the method of the invention heats the crystal to a temperature ashigh as practical to produce as many states as possible in the crystalwhile avoiding or minimizing degradation or sublimation of the crystal.

During the heating step, the crystal is preferably maintained at theelevated temperature for a period of at least about two minutes, aninterval that has practical and functional considerations. From apractical standpoint, it will take several minutes under mostcircumstances to heat the silicon carbide crystal to this temperature.From a functional standpoint, this also provides sufficient time for thecrystal to reach an equilibrium or near equilibrium condition withrespect to the states that are desirably generated. The heating time ispresently functionally best expressed as a time sufficient to obtain athermal equilibrium or near equilibrium in the crystal having thedesired number of states. It will be understood that the crystal doesnot need to reach a full equilibrium in the most proper or restrictedsense of that term, but the term is used herein to describe a conditionin which the crystal reaches a given temperature and is maintained therefor a time sufficient to develop the desired number of states.

The step of heating the crystal preferably comprises heating the crystalin an induction heater, in which case the step of cooling the crystalincludes (at least) reducing the power to the induction coil. Inductionheaters and their method of operation in semiconductor manufacture aregenerally well understood in the art and can be incorporated accordingto the invention without undue experimentation. Thus, as the particularinduction heater is not critical to the claimed invention, it will notbe discussed in detail otherwise herein. Additionally, other types ofheating can be used by those of ordinary skill in this art and withoutundue experimentation.

Once the crystal has been heated for the desired period of time at thetemperature of 2000° C. or above, it is cooled in a manner that avoidsdwelling for any significant time period in the temperature range wherethe defects are mobile enough to disappear or to be re-annealed into thesystem. In presently preferred embodiments, a rate of cooling in excessof about 30° C. per minute appears to be preferred with a rate of 150°C. per minute appearing to be a useful upper limit.

It will be understood by those familiar with thermodynamics and theheating and cooling of materials, particularly materials at relativelyhigh temperatures, that the rate of cooling need be neither constant norexact throughout the entire cooling process. Stated differently, whilethe crystal is cooling, and particularly while it is cooling withintemperature ranges where re-annealing can occur at significant rates,the rate of cooling should desirably range between the 30° C. per minuteand 150° C. per minute preferred limits. For the usual andwell-understood thermodynamic reasons, the heat loss and thus the rateof cooling will tend to be most rapid as the crystal cools from thehighest temperatures and will tend to moderate as the crystal approachesand reaches lower temperatures. In particular, once the crystal iscooled below the temperature range in which re-annealing takes place atsignificant rates, the rate of cooling can become slower without anyfunctional disadvantage. Accordingly, as an individual crystal iscooled, the rate at which it cools can vary within the 30°-150° C. perminute preferred range while still taking advantage of the method of theinvention.

A rate of cooling that is too slow allows the crystal to spend too muchtime in the temperature range at which the states will heal and thecrystal become sufficiently ordered to reduce the number of states belowthe number necessary to retain the semi-insulating characteristics.Alternatively, cooling at an overly-rapid rate can produce mechanicalstresses in the crystal including fracturing if the thermal stress issufficiently great.

In preferred embodiments the cooling step includes both passive andactive steps. As a first step, the power to the induction heater iseither reduced or turned off entirely. At the relatively hightemperatures to which the crystal has been heated, the initial heat lossis a radiation heat loss. As the temperature becomes lower, themechanisms of conduction and convection cooling take over. Accordingly,to further encourage and control the cooling rate, the heating chambercan be flooded with an inert gas, typically argon. Additionally, thethermal mass of the crystal and of the materials with which it is placedin contact can be used to help control the cooling rate. Consequently,three basic ways to control the rate of cooling include adjusting thepower to the induction coil (or to any other relvant heating mechanismwell understood in this art such as resistance heating); flowing acooling gas around and over the silicon carbide crystal; and controllingthe thermal mass of the crystal and its surroundings; i.e. such as theuse of a heat sink. Because these are thermodynamic conditions, they canbe addressed in a number of different ways that are not critical to theclaimed invention and can be carried out by those of ordinary skill inthis art without undue experimentation.

The preferred cooling rate of between about 30° and 150° C. per minutecan be also expressed as cooling the crystal to about room temperaturein less than about 70 minutes, or—at a more rapid pace—cooling thecrystal to about room temperature in less than about 20 minutes.

Because the invention provides a beneficial semi-insulating siliconcarbide crystal, including substrate wafers, the method of the inventioncan further comprise the steps of heating the silicon carbide substratewafer to a temperature of about 2,000° C. (and preferably to between2,000° and 2,400° C.), cooling the heated wafer to approach roomtemperature at a rate of at least about 30° C. per minute (andpreferably approaching 150° C. per minute), and then depositing one ormore epitaxial layers of semiconductor material on the substrate wafer.Because the advantage of silicon carbide often (although notexclusively) relates to its wide bandgap properties, in preferredembodiments, the step of depositing the epitaxial layer will comprisedepositing an epitaxial layer selected from the group consisting ofother wide bandgap semiconductors such as silicon carbide or Group IIInitrides using chemical vapor deposition (CVD) techniques. In the caseof silicon carbide, the step of depositing the epitaxial layer istypically carried out at temperatures greater than about 1,400° C. Asnoted above, in prior techniques steps carried out at such temperaturestended to reduce the number of defects to a point at which the substratewould no longer have appropriate semi-insulating characteristics.Because the invention provides a method for controllably increasing thenumber of point defects and resulting deep level states as compared to“as-grown” crystals, these later processing steps do not spoil thesemi-insulating character of the crystal even though some of the defectsare expected to heal.

Accordingly, in another aspect, the invention comprises the wafer andepitaxial layer(s) produced by this aspect and embodiment of theinvention.

The invention can be carried out on substrate wafers or single crystalboules, with substrates being the preferred embodiment because theirlarge surface-to-volume ratio enables them to cool at the relativelyrapid rates that are useful in the invention without suffering undue orcatastrophic thermal stress. Other than this practical point, however,there is no conceptual difference between the manner in which theadditional states can be created in a wafer versus a boule. Accordingly,the invention can also comprise the steps of heating a silicon carbideboule to a temperature of at least about 2,000° C., then cooling theheated boule to approach room temperature at the rate of at least about30° C. per minute, then slicing a silicon carbide wafer from the bouleand then depositing one or more epitaxial layers of semiconductormaterial on the sliced wafer.

In an alternative embodiment, the method can comprise the steps ofslicing the silicon carbide wafer from the single crystal boule, thenheating the sliced wafer to the temperature of at least about 2,0000degrees C. at atmospheric pressure, and then cooling the heated wafer toapproach room temperature at a rate of at least 30° C. per minute, andthereafter depositing the epitaxial layer(s) of semiconductor materialon the sliced wafer.

As known to those familiar with the preparation of substrate wafers andthe growth of epitaxial layers, the sliced silicon carbide wafer isgenerally not used immediately after having been sliced, but instead iscleaned and polished to prepare a more favorable surface for epitaxialgrowth. The polishing and cleaning steps for semiconductor materials ingeneral and silicon carbide in particular are well established in thisart, can be practiced without undue experimentation, and will not beotherwise discussed in detail herein.

In either case, the invention further comprises the wafer and one ormore epitaxial layers, and can further comprise devices that incorporatethe wafer and epitaxial layers formed according to the methods of theembodiments of the invention.

The invention is not limited to use with any particular devices, butcommonly used microwave devices that incorporate semi-insulating siliconcarbide substrates include various types of field effect transistors(FETs), metal oxide semiconductor field effect transistors (MOSFETs),junction field effect transistors (JFETs), metal-semiconductor fieldeffect transistors (MESFETs), heterostructure field effect transistors(HFETs), high electron mobility transistors (HEMTs), and DMOStransistors. Those familiar with semiconductor devices and devicesuseful for microwave frequency operation will recognize that this listis neither limiting nor exhaustive. It is, however, illustrative of theadvantages provided by the invention described and claimed herein.

FIG. 1 schematically illustrates the temperature ranges and the coolingrates used in the present invention. Those familiar with thesetechniques will recognize that FIG. 1 is explanatory in nature ratherthan an exact representation of particular experiments.

FIG. 1 is a plot of temperature versus time. Three general sets oftemperatures are characterized. The uppermost line designated at 10represents the temperature, preferably 2,000° C. for silicon carbide,above which the desired number of point defects is produced in themanner according to the present invention. Stated differently, theinvention includes the step of heating the silicon carbide crystal tothe temperature represented by line 10 or higher.

The second highest line is designated at 12 and represents a lowertemperature (which will be understood as relative rather than exact, butwhich in preferred embodiments is about 1200° C.) that together with theupper temperature line 10 defines a temperature range (represented bythe arrow 11) within which the states created above the temperature line10 will be expected to heal if the crystal is allowed to remain in thistemperature range for a period of time sufficient to approach anequilibrium or a near equilibrium condition. Accordingly, the inventionas described herein minimizes the time that the crystal spends in thetemperature range 11 once the increased number of states has beenproduced. As noted above, maintaining the cooling rate at between about30° C. and 150° C. per minute is particularly helpful while the crystalis within the temperature range schematically illustrated at 11 in FIG.1.

The third line designated at 14 represents room temperature (25° C., 298K) and defines another temperature range (designated by the arrow 13)between room temperature and the temperature line 12. The temperaturerange symbolized by the arrow 13 represents temperatures that are stillabove room temperature, but within which the amount of reordering thatmay occur is statistically insignificant to the semi-insulatingcharacteristics.

For any number of reasons, the crystal normally can be expected to coolall the way to room temperature whether during pre-manufacture, storage,shipping or even use. It will be understood, however, that provided thecrystal is heated to a temperature above that represented by the line10, and then cooled sufficiently rapidly to a temperature below thetemperature represented by the line 12, the benefits of the inventionwill be accomplished, regardless of whether room temperature is everreached.

Three cooling curves are schematically illustrated as the lines at 15,16, and 17. Because FIG. 1's abscissa represents time, it will beunderstood that the line 15 represents the slowest rate of cooling,while the line 17 represents the most rapid. In this sense, the extendedcurve 15 illustrates that the crystal would spend a much greater periodof time in the temperature range designated by the arrow 11 as comparedto crystals following the cooling curves designated by the lines at 16or 17. Thus, the curve 15 schematically represents a prior art approach(intentional or unintentional) to cooling the crystal, while the lines16 and 17 schematically represent the more rapid cooling steps of thepresent invention. As noted previously, provided the cooling rate meetsthe functional aspects described herein, the rate need not be constant.

FIG. 2 illustrates that the desired high concentrations of deep-levelscorrelate with higher growth temperatures. FIG. 2 plots the change incapacitance as measured by deep level transient spectroscopy (DLTS)against temperature. The higher amplitude (e.g., at 300 K) of thecrystal samples grown at higher temperatures (solid line) represents alarger concentration of deep levels as compared to a sample grown at alower temperature (dashed line).

Deep level transient spectroscopy is generally well understood in thesemiconductor arts and is a sensitive method used to study deep levelsin semiconductors. The method is based on the capacitance charge of areversed biased diode when deep levels emit their carriers after theyhave been charged by a forward bias pulse. The emission rate istemperature dependent and characteristic for each type of defect. Usingthe temperature dependence of the emission rate, the activation energyof a deep level can be determined. See, e.g., ASTM International TestNo. F978-02, “Standard Test Method for Characterizing Semiconductor DeepLevels by Transient Capacitance Techniques.” Other techniques forevaluating the crystal can include capacitance versus voltage (CV)techniques, as well as electron paramagnetic resonance (EPR).

FIG. 3 is a comparative set of three plots from electron paramagneticresonance (EPR) evaluation of silicon carbide crystal samples. EPR is awell-understood technique for measuring certain characteristics ofmaterials and is also known as electron spin resonance (ESR) or electronmagnetic resonance (EMR). EPR represents the process of resonanceabsorption of microwave radiation by paramagnetic ions or molecules,with at least one unpaired electron spin and in the presence of amagnetic field. In analyzing crystals according to the presentinvention, EPR is used to measure the number of charges occupying deeptraps in the crystal bandgap. By measuring the change in absorption ofmicrowave energy within a continuously varying strong magnetic field,EPR detects the number of unpaired spins of electronic charges trappedat various defects in the crystal lattice. The EPR measurement does not,however, evict the charges from the traps, but merely detects theirpresence, thus permitting repeated analysis of the same sample.

The three plots of FIG. 3 represent (from left to right), a siliconcarbide crystal grown conventionally, a silicon carbide crystal heatedand cooled in the range of 30° C. per minute according to the presentinvention and a crystal heated and cooled in the range of 150° C. perminute according to the present invention.

Each of the sections of FIG. 3 is sized identically and the magnitude(arbitrary units) of the EPR signal of the carbon vacancy (V_(c))—i.e.one of the types of point defects that provides the states that in turnprovide semi-insulating character—is proportional to the number ofdefect centers detected by the EPR. As known to those familiar with EPR,the “g-factor” (or “g-value”) is characteristic of the type of electrontrap and is related to the microwave frequency and the magnetic fieldstrength. Accordingly, given that the sample sizes measured were thesame within expected margins of experimental error, the magnitude of theEPR line for the carbon vacancies (from the trough to the peak) isproportional to the concentration of defects in the sample. Thus, FIG. 3illustrates a significant increase in the number of carbon vacancies(and a resulting improvement in semi-insulating character) from theas-grown condition (left panel) to the process of the invention using a30° C. rate of cooling (middle panel) to the process of the inventionusing a 150° C. rate of cooling (right hand panel).

In another aspect, the invention is a method of reducing the nitrogencontent of a semi-insulating silicon carbide crystal grown bysublimation and a resulting high purity semi-insulating silicon carbidecrystal with a reduced nitrogen concentration therein. One embodiment ofthe invention introduces a hydrogen or hydrogen-containing ambientatmosphere into a sublimation growth chamber used to grow siliconcarbide crystals. Previously standard sublimation systems utilize anargon ambient in the growth of silicon carbide. The inventors hereinhave discovered that a hydrogen ambient is more useful than otherambient gases to control the nitrogen content of the growing crystal.

The method of the invention herein includes introducing a siliconcarbide source powder and a silicon carbide seed crystal into asublimation growth chamber. The source powder, as its name implies,provides a source of silicon carbide species in the growth chamber forgrowing a silicon carbide crystal on a growth surface provided by thesilicon carbide seed crystal. U.S. Pat. No. Re. 34,861, the entirecontents of which are incorporated by reference herein, sets forth thatsolid silicon carbide in powdered form is one such preferred sourcematerial. The method of the first embodiment includes heating thesilicon carbide source powder to sublimation in a hydrogen ambientgrowth chamber. The hydrogen ambient of the sublimation growth chamberis established by introducing hydrogen gas into the growth chamber at apressure of between about 0.1 and 50 Torr and at a flow rate of betweenabout 10 and 1000 standard cubic centimeters per minute (sccm).

The sublimation process requires temperature control of differentregions within the growth chamber. While heating the silicon carbidesource powder to a first temperature, the silicon carbide seed crystalis heated and maintained at a second temperature approaching thetemperature of the source powder. The temperature of the seed crystalis, therefore, lower than the temperature of the source powder and lowerthan that temperature at which silicon carbide will sublime. The reducedseed crystal temperature encourages sublimed species from the sourcepowder to condense upon the seed crystal. The seed crystal, therefore,provides the growth surface for fabricating a silicon carbide crystalwith desired dimensions. The method herein includes a continued heatingof the silicon carbide source powder until a desired amount of siliconcarbide crystal growth has occurred upon the seed crystal.

The seed crystal preferably has a polytype selected from among the 3C,4H, 6H and 15R polytypes of silicon carbide, depending on the polytypedesired in the resulting grown crystal. The silicon carbide species thatcondense onto the seed crystal optimally grow a silicon carbide crystalwith the same polytype as the seed crystal.

The invention maintains a thermal gradient between the growth surface ofthe seed crystal and the source powder. Re. 34,861 describes variousmeans for maintaining a thermal gradient between the source powder andthe seed crystal. The gradient may be accomplished, for example, byestablishing a desired geometric distance and temperature differencebetween the seed crystal and the source powder. Otherwise, thetemperature gradient may be established by independently controlling thetemperatures of the respective regions within the growth chamber inwhich the silicon carbide powder sublimes and the silicon carbidecrystal grows.

Typically, the silicon carbide source powder is maintained at atemperature of between about 2000° C. and 2500° C. The seed crystal, inturn, is maintained at a temperature of between about 50° C. and 350° C.lower than the temperature of the source powder.

The method herein further includes maintaining the source powder and theseed crystal during sublimation growth at respective temperatures highenough to increase the number of point defects in the growing crystal.The point defects, in conjunction with lower nitrogen content, renderthe resulting silicon carbide crystal semi-insulating.

Alternatively, the step of increasing the number of point defects cancomprise the previously described method of heating a silicon carbidecrystal to a temperature above the temperatures required for CVD growthof silicon carbide from source gases, but less than the temperatures atwhich disadvantageously high rates of silicon carbide sublimation occurunder the ambient conditions to thereby thermodynamically increase theconcentration (i.e., number per unit volume) of point defects andresulting states in the crystal; and then cooling the heated crystal toapproach room temperature at a sufficiently rapid rate to minimize thetime spent in the temperature range in which the defects aresufficiently mobile to disappear or be re-annealed into the crystal tothereby produce a silicon carbide crystal with a concentration of pointdefect states that is greater than the concentration of point defectstates in an otherwise identically grown silicon carbide crystal thathas not been heated and cooled in this manner.

The method described and claimed herein allows the production ofsemi-insulating silicon carbide crystals without the need for deep leveldoping elements, such as vanadium. As discussed in commonly assignedU.S. Pat. No. 6,218,680, prior art methods of fabricatingsemi-insulating silicon carbide incorporate dopants that form energystates at levels between the valence and conduction bands of siliconcarbide. These energy states of the prior art are far removed from boththe conduction band and the valence band to enhance the semi-insulatingqualities of the crystal. Common deep level trapping elementsincorporated into silicon carbide include vanadium and other transitionmetals. The method herein allows the fabrication of semi-insulatingsilicon carbide crystals without relying upon more complicated dopinglevels that may be quite difficult to accurately control.

The method developed by the inventors herein allows for deep leveltrapping elements in the silicon carbide source powder to be kept tominimum levels, thereby simplifying the fabrication process. The deeplevel trapping elements in the silicon carbide source powder arereferred to herein as being present in amounts that are “belowdetectable levels,” meaning that the elements are present in amountsthat cannot be detected by modern sophisticated analytical techniques.

In particular, because one of the more common techniques for detectingelements in small amounts is secondary ion mass spectroscopy (“SIMS”),the detectable limits referred to herein are those amounts of elementssuch as vanadium and other transition metals that are present in amountsless than 1×10¹⁶ (1E16), or in other cases (including vanadium), lessthan 1E14. These two amounts represent typical detection limits for mosttrace elements (particularly vanadium) using SIMS techniques; e.g., SIMSTheory-Sensitivity and Detection Limits, Charles Evans & Associates(1995), www.cea.com.

The method described and claimed herein helps control the amount ofnitrogen incorporated into the growing silicon carbide crystal bycontrolling the hydrogen concentration in the ambient atmosphere of thegrowth chamber. Although the inventors do not wish to be bound by anyparticular theory, the effectiveness of the hydrogen on suppressing thenitrogen in the crystal is attributed to the passivation of the siliconcarbide growth surface by hydrogen atoms. The hydrogen atoms, in effect,block, reduce, or otherwise hinder the incorporation of nitrogen atomsat the surface of the growing crystal.

The method of the invention herein, therefore, is appropriatelydescribed in another embodiment as a method of passivating a growingsilicon carbide crystal in a sublimation growth chamber to control thenitrogen that can be incorporated into the crystal. A second embodimentof the invented method includes introducing an ambient gas containinghydrogen into the growth chamber and heating a silicon carbide sourcepowder to sublimation in the hydrogen ambient growth chamber. The sourcepowder is heated while simultaneously heating and maintaining a siliconcarbide seed crystal in the hydrogen ambient growth chamber to a secondtemperature below the temperature of the source powder. The temperatureof the seed crystal is low enough for sublimed species from the sourcepowder to condense upon the seed crystal.

The hydrogen passivation method, furthermore, maintains the sourcepowder and the seed crystal during sublimation growth at respectivetemperatures high enough to increase the number of point defects in thegrowing crystal. The point defects assist in rendering the resultingsilicon carbide crystal semi-insulating. The heating, sublimation, andcondensation steps continue at sufficiently high temperatures to inducea sufficient number of point defects for a semi-insulating crystal. Theoverall silicon carbide deposition continues until a desired amount ofhighly pure semi-insulating silicon carbide crystal grows upon the seedcrystal.

An ambient concentration of hydrogen is maintained in the growth chambersufficient to passivate the growing silicon carbide crystal against theincorporation of nitrogen. The hydrogen passivation thereby controls theamount of nitrogen incorporated into the growing silicon carbidecrystal. The inventors do not wish to be bound by any particular theory,but research in the field of sublimation grown silicon carbide crystalssuggests that the hydrogen atoms in the ambient atmosphere of the growthchamber reduce the number of unpaired electrons in the silicon carbidecrystal. U.S. Pat. No. 5,151,384, issued to Williams and incorporated byreference herein, describes and claims the electron pairing of hydrogenpassivation on silicon compounds at column 2, lines 38-70. Thisreduction of unpaired electrons corresponds to a reduced number ofnitrogen atoms likely to bond with the silicon carbide crystal as itgrows.

Alternative explanations also exist to explain the physical mechanism bywhich the hydrogen ambient suppresses nitrogen incorporation. Oneexplanation is that the hydrogen passivation of the silicon carbidecrystal is essentially a layer of hydrogen atoms on the crystal growthsurface that protects the surface from nitrogen incorporation. See,e.g., U.S. Pat. No. 5,709,745 (col. 26, lines 18-24); U.S. Pat. No.6,113,451 (col. 8, lines 38-44); and U.S. Pat. No. 6,201,342 (col. 8,lines 33-39), all of which are incorporated entirely by referenceherein.

Finally, published European Patent Application 0561462A2 (col. 10, lines42-48) filed on Mar. 12, 1993 describes hydrogen passivation as fillingin spaces between the silicon carbide crystal grain boundaries anddisallowing nitrogen incorporation therein. The inventors do not rely onany particular one of these descriptions of the effects of hydrogen insilicon carbide growth. The method disclosed and claimed hereinsuccessfully controls nitrogen content by a combination of thesephysical and chemical interactions between the hydrogen atoms and thegrowing silicon carbide crystal. The method results in a desirablehighly pure, semi-insulating silicon carbide crystal growth.

Controlling the hydrogen flow rate into the growth chamber between about80 and 1000 standard cubic centimeters per minute (sccm) at a pressureof between about 0.1 and 50 Torr provides a sufficient hydrogenconcentration in the growth chamber to yield the desired crystal. Themethod has proven successful in fabricating a silicon carbide crystalwith less than about 2×10¹⁵ nitrogen atoms per cubic centimeter (cm⁻³).In preferred practice, the hydrogen concentration in the ambientatmosphere yields a silicon carbide crystal with less than about 1×10¹⁵cm⁻³ nitrogen atoms. The low nitrogen concentration of the resultingsilicon carbide crystals, coupled with the deep levels, yields aresistivity greater than or equal to 1×10⁵ ohm-cm.

The invention claimed herein is useful in a variety of otherapplications. For example, synthesizing high purity silicon carbidepowder in a hydrogen containing environment potentially reduces nitrogencontent in the source powder. The technique is also useful in themanufacture of near colorless gemstone material. Finally, the control ofnitrogen incorporation in a silicon carbide crystal represents anadvancement in the manufacture of semi-insulating crystals and wafers ofsilicon carbide used in the manufacture of MESFET and HEMT highfrequency electronic devices. The technique disclosed herein provides anefficient and straight forward method of achieving extremely lownitrogen levels in silicon carbide crystals and wafers. In fact, bulkwafers having extremely low concentrations of nitrogen may be producedby controlling the nitrogen content of a growing crystal. Wafers madefrom these crystals may displace the need for the thick high purityepitaxial layers grown on current silicon carbide substrates.

FIGS. 4 and 5 illustrate that the invention described and claimed hereinpresents a significant advancement in the field of growing highly pure,semi-insulating silicon carbide crystals by sublimation. The hydrogenambient controls the nitrogen incorporated into the crystal and providesa more reliable semi-insulating silicon carbide crystal quality. FIG. 4shows the low temperature photoluminescence spectrum corresponding to a4H-silicon carbide crystal grown in a conventional argon ambient. FIG. 5shows the low temperature photoluminescence spectrum corresponding to a4H-silicon carbide crystal grown in a hydrogen ambient according to theinvention herein.

A background discussion is helpful in understanding the advantagesillustrated by FIGS. 4 and 5. The figures plot the photoluminescencespectra for silicon carbide crystals and show luminescent intensitypeaks at specific wavelengths. These peaks of luminescence areproportionally related to the nitrogen content of the silicon carbidecrystal under consideration. See Ivanov et al., Nitrogen DopingConcentration as determined by Photoluminescence in 4H- and 6H-SiC,Journal of Applied Physics, vol. 80, no. 6, Sep. 15, 1996, pp.3504-3508. The nitrogen concentration in a crystal can be determined bythe luminescence of electrons and holes during their recombination atneutral nitrogen centers.

In the study of electron-hole recombinations, silicon carbide is knownas an indirect bandgap semiconductor. As known to those familiar withelectronic transitions, a direct transition occurs in a semiconductorwhen the valence band maxima and the conduction band minima have thesame momentum state. This means that crystal momentum is readilyconserved during recombination of electrons and holes so that the energyproduced by the transition can go predominantly and efficiently into thephoton, (i.e., to produce light rather than heat). When the conductionband minimum and valence band maximum do not have the same momentumstate, a phonon (i.e., a quantum of vibrational energy) is required toconserve crystal momentum and the transition is called “indirect.” Thenecessity of a third particle, the phonon, makes indirect radiativetransitions less likely, thereby reducing the light emitting efficiencyof the crystal.

The indirect band gap of silicon carbide prevents the directrecombination of holes and electrons. The direct non-phonon assistedrecombination of a free exciton, independent of other particles, istherefore forbidden. The recombination of electrons and holes in siliconcarbide requires the formation of the previously discussed phonon toaccount for the difference in momentum between recombined electrons andholes.

Ivanov et al. reported in 1996 that the electron-hole exciton may becoupled to a phonon or bound to an impurity in the crystal to accountfor the required conservation of momentum. The luminescence intensity ofthe recombination is dependent upon whether the recombined electron-holepair is bound to a phonon or to an impurity, such as nitrogen. SeeIvanov et al., supra., pp. 3504-3508. Ivanov et al., therefore, showthat the concentration of impurity in a crystal can be determined bycomparing the luminescence intensity of an electron-hole recombinationpaired with an impurity and the luminescence intensity of anelectron-hole recombination paired with a phonon.

FIGS. 4 and 5 herein illustrate these concepts and show the success ofthe nitrogen reduction method of the present invention. The figures plotthe relative luminescence intensity versus wavelength for 4H siliconcarbide crystals. The peak luminescence intensity is shown as Q0 andcorresponds to the intensity of an electron-hole recombination bound toa nitrogen atom as an impurity in the crystal. Less intense peaks ofluminescence in the figures correspond to phonon coupled recombinations,the most significant of which for purposes herein is the recombinationmarked I75. I75 is the highest intensity phonon-assisted recombinationand can be identified by its asymmetric line shape (Ivanov, supra at3505). As known to those in the art, the ratio of Q0 to I75 yields aconstant that can be used to extrapolate the nitrogen content of thesubject silicon carbide crystal (Ivanov, supra at 3508).

Considering FIG. 4, the luminescence intensity is plotted for a 4Hsilicon carbide crystal grown by sublimation in a traditional argonambient atmosphere. FIG. 4 is, therefore, indicative of prior art in thearea of silicon carbide grown by sublimation. The extrapolated nitrogencontent is approximately 3×10¹⁵ nitrogen atoms per cubic centimeter ofthe resulting silicon carbide crystal.

FIG. 5 shows the luminescence data corresponding to a crystal grown in ahydrogen ambient atmosphere by the invention disclosed herein. As can beseen in the spectrum, the ratio of Q0 to I75 is at 0.6, corresponding toa nitrogen concentration in the crystal of 3×10¹⁴ nitrogen atoms percubic centimeter. The data of FIG. 5 shows that the presence of ahydrogen ambient in the sublimation growth chamber reduced the nitrogencontent in the crystal by approximately one order of magnitude. FIG. 5shows, therefore, that the hydrogen concentration in the growth chamberambient can be used to reduce the nitrogen content of the resultingsilicon carbide crystal grown therein. The resulting low nitrogencrystal is a high purity semi-insulating crystal as desired.

In yet another aspect, the method of the present invention helps tocontrol the amount of nitrogen incorporated into the growing siliconcarbide crystal by controlling the hydrogen concentration in the ambientatmosphere of the growth chamber. The hydrogen atoms, in effect, block,reduce, or otherwise hinder the incorporation of nitrogen atoms at thesurface of the growing crystal. Although the inventors do not wish to bebound by any particular theory, the effectiveness of the hydrogen oncontrolling the nitrogen in the crystal is attributed to two principalmechanisms. The first possible mechanism induced by the presence of thehydrogen is a reduction in the number of carbon-vacancy sites wherenitrogen atoms can reside in the SiC crystal lattice. This reduction incarbon vacancies derives from a shift in the crystal composition towardcarbon-richness due to the effect of hydrogen on the silicon to carbonratio of molecular species emanating from the SiC sublimation source.[This mechanism is described from a theoretical point of view in severalpapers treating the thermodynamics of the SiC-H2 system (see forexample: Lilov et al., J. Crystal Growth 32 (1976) 170, Studies ofGrowth Processes in Silicon Carbide Epitaxial Layers from the VaporPhase; and Rabeck et al., J. Electrochem. Soc. 144 (1997) 1024Thermodynamic Considerations of the Role of Hydrogen in SublimationGrowth of Silicon Carbide)]. Since nitrogen is incorporated on thecarbon sub-lattice at carbon-vacancy sites, the amount of incorporatednitrogen is reduced when the concentration of carbon vacancies isreduced.

The second possible mechanism is a passivation mechanism in which thesilicon carbide growth surface is directly passivated by the hydrogenatoms thereby preventing or otherwise hindering the incorporation of thenitrogen.

The method of the invention herein, therefore, is appropriatelydescribed in another embodiment as a method of passivating a growingsilicon carbide crystal in a sublimation growth chamber to control thenitrogen that can be incorporated into the crystal. A second embodimentof the invented method includes introducing an ambient gas containinghydrogen into the growth chamber and heating a silicon carbide sourcepowder to sublimation in the hydrogen ambient growth chamber. The sourcepowder is heated while simultaneously heating and maintaining a siliconcarbide seed crystal in the hydrogen ambient growth chamber to a secondtemperature below the temperature of the source powder. The temperatureof the seed crystal is low enough for sublimed species from the sourcepowder to condense upon the seed crystal. The heating, sublimation, andcondensation steps are continued until a desired amount of siliconcarbide crystal growth has occurred upon the seed crystal. An ambientconcentration of hydrogen is maintained in the growth chamber sufficientto passivate the growing silicon carbide crystal against theincorporation of nitrogen and to thereby control the amount of nitrogenincorporated into the growing silicon carbide crystal.

The inventors do not wish to be bound by any particular theory, butresearch in the field of sublimation grown silicon carbide crystalssuggests that the hydrogen atoms in the ambient atmosphere of the growthchamber reduce the number of unpaired electrons in the silicon carbidecrystal. U.S. Pat. No. 5,151,384, issued to Williams and incorporated byreference herein, describes and claims the electron pairing of hydrogenpassivation on silicon compounds at column 2, lines 38-70. This type ofreduction of unpaired electrons corresponds to a reduced number ofnitrogen atoms likely to bond with the silicon carbide crystal as itgrows.

The hydrogen ambient may be established within the growth chamber by thedirect flow of hydrogen, as noted above. Alternatively, a hydrocarbonspecies, such as methane, may be introduced into the growth chamberinstead of pure hydrogen. A similar effect on nitrogen reduction in thegrowing crystal occurs because the hydrocarbon species tend to crack athigh growth temperatures used during sublimation. The crackedhydrocarbon species principally produce hydrogen, which would then havethe same effects as a pure hydrogen ambient in the growth chamber. Anyhydrocarbon can serve this purpose provided it supplies the hydrogenwithout otherwise interfering with the source powder, the seed, thegrowing crystal, the growth process or the equipment.

In yet another aspect, the invention can be described in terms of thesublimation growth of silicon carbide, but is not limited to siliconcarbide per se.

As noted in the background portion of the specification, the generalaspects of seeded sublimation growth of silicon carbide have beengenerally well established for a number of years. Furthermore, thosefamiliar with the growth of crystals, particularly in difficult materialsystems such as silicon carbide, will recognize that the details of agiven technique can and will vary, usually purposefully, depending uponthe relevant circumstances. Accordingly, the descriptions given hereinare most appropriately given in a general and schematic sense with therecognition that those persons of skill in this art will be able tocarry out the improvements of the invention based on the disclosuresherein without undue experimentation.

FIG. 6 is a micrograph of a micropipe-caused failure in a siliconcarbide device (V. Dmitriev et al., Large Area Silicon Carbide DevicesFabricated on SiC Wafers with Reduced Micropipe Density, MaterialsScience and Engineering B61-62 (1999) 446-449).

FIG. 7 is another micrograph of a micropipe as set forth by Bakin etal., State of the Art in Defect Control of Bulk SiC, Invited Talk at the1998 High Temperature Electronic Materials, Devices and SensorsConference (1998), San Diego, Calif., pp. 2-13. FIGS. 1 and 2 areincluded for illustration purposes and to provide context to theinvention.

As noted in the Background, recent thinking hypothesizes that unintendedsublimation from the rear (opposite to growth) face of the seed crystalinitiates micropipe defects that propagate through the seed andthereafter into the growing crystal. From a functional standpoint, thegoal is to maintain a proper growth shape throughout the entire growthcycle while minimizing or eliminating relevant defects such asmicropipes.

Although such effects are addressed herein, the present invention alsoincludes the recognition that other (or in some cases most) micropipedefects do not necessarily initiate in the seed, but rather in thegrowing crystal during the early stages of its growth. In turn, theinvention incorporates the recognition that the physical relationshipbetween and among the seed crystal and the seed holder similarly affectsthe degree to which defects form—or are prevented from forming—in thegrowing crystal.

In turn, the invention incorporates the recognition that inconsistentthermal circumstances at the interface of the seed crystal and the seedholder tend to generate changes in growth that can be, and often are,manifested as micropipe defects in the growing crystal. Accordingly, theinvention provides various techniques for ensuring a consistent thermalrelationship between the seed crystal and the seed holder.

Stated differently, if the temperature profile across the seed-seedholder interface is inconsistent, such thermal differences canconductively affect the growth interface between the seed and thegrowing crystal. The resulting thermal inconsistencies at theseed-growth interface have been found to encourage the initiation andpropagation of micropipe defects in the growing bulk crystal.

In one aspect of the invention, providing the consistent thermalcircumstances is achieved by minimizing the separation of the seed andthe seed holder to ensure that conductive heat transfer exceeds, andpreferably dominates, radiative heat transfer over substantially theentire area between the seed and the seed holder. In particular, it hasbeen discovered according to the present invention that superior resultsare achieved when the seed crystal and seed holder are in macroscopicdirect contact and that their respective contacting surfaces alwaysconform to one another within 10 μm or less, more preferably 5 μm orless, and most preferably 2 μm or less, including less than one μmwherever possible.

In turn, such a relationship can be fostered by using a seed crystalthat deviates from flat by no more than 10 μm, more preferably no morethan 5 μm and most preferably less than 2 μm and potentially less than 1μm.

Furthermore, using a seed holder that deviates from flat by no more than10 μm is likewise preferable to obtain the desired relationship. As inthe case of the seed, the deviation is preferably no more than 5 μm andmost preferably less than 2 μm and potentially less than 1 μm.Additionally, the seed crystals should be internally homogeneous.

These tolerances are at least an order of magnitude more accurate thanthose generally recognized in the art as being necessary for suchpurposes. For example, a silicon wafer of three-inch (3″) diameter isconsidered acceptable if it has a total thickness variation (TTV) of 25μm, a maximum bow of 40 μm, and a maximum warp of 40 μm (e.g., SEMIM1.2-89, “Standard for 3 inch Polished Monocrystalline Silicon Wafers,SEMI 1978, 1999).

As used herein, the term “bow” refers to the bending of the wafer into acurve; i.e., the deviation of the center point from the average positionof the surface. In slight contrast, the term “warp” refers to a twistingor turning of the wafer away from its desired flat shape; i.e., thelargest “peak” to “valley” distance across the wafer.

Obtaining the desired conformity between the seed and the seed holdercan be enhanced by a number of techniques. It will be understood thatfrom a theoretical perspective, the seed and seed holder can be anyrespective shapes provided that they conform within the 10 μm tolerance.In most circumstances, however, the most straightforward method ofobtaining the desired conformity is to make both items as flat aspossible.

Thus, in one aspect the method comprises using a relatively thick seedcrystal; i.e., on the order of about 1.4 mm rather than the moreconventional 600-700 μm for a three-inch seed crystal. The thicker seedoffers several advantages in the method of the invention. First, becauseincreased thickness correspondingly increases the mass of the seed, theseed has a greater total heat capacity and can more efficiently moderatetemperature differences that originate at the seed holder and presentthe growing seed with a lesser thermal inconsistency than exists at theseed-seed holder interface. The high thermal conductivity of siliconcarbide also helps moderate temperature differences more quickly andevenly in larger (i.e., proportionally thicker) seeds than in thinnerones.

Second, a thicker seed can be polished more readily. Polishing the seedoffers at least two advantages in the invention. First, it helps reachthe flatness tolerances noted above. Second, polishing helps reduce oreliminate subsurface damage that may be present in the seed crystal.According to the invention, it has been discovered that if suchsubsurface damage remains in the seed, the high growth temperatures usedfor silicon carbide will tend to anneal (repair) such damage at hightemperature. The annealed crystal then tends to experience mechanicalrelaxation in the absence of the mechanical stress caused by thesubsurface damage at room temperature. In turn, the relaxed crystaltends to change shape, which in turn tends to produce the undesired andproblematic thermal inconsistencies.

Stated differently, if the seed is flat at room temperature in thepresence of the stress produced by any subsurface defects, annealing theseed will relax the stress and thus cause the wafer to deviate fromflat.

As a third advantage, at any given diameter a thicker seed simply tendsto bend less at any temperature, including growth temperatures, allother factors being equal.

In another aspect, the method can comprise double side lapping of theseed wafer to machine it to a flat shape exhibiting a deviation fromflat of less than 10 μM, more preferably less than 5 μm and mostpreferably 2 μm or less. Alternatively, the wafer can be machined to ashape conforming to the shape of the seed holder for the same purpose.Other equivalent methods can also be incorporated such as grinding orpolishing. As stated above, the goal is to produce a seed over whichthere is no more than 10 μm deviation from flat at any point.

FIG. 8 is a cross sectional schematic diagram of a sublimation systemfor seeded sublimation growth of the type for which the inventionprovides several improvements. The system is broadly designated at 20.As in most typical systems, the system 20 includes a graphite susceptor21 and a plurality of induction coils 22 that heat the susceptor 21 whencurrent is applied through the coils 22. Alternatively, some systemsincorporate resistance heating. It will be understood by those familiarwith these crystal growth techniques that the system can be furtherenclosed in some circumstances, e.g., in a water-cooled quartz vessel.Such further enclosures are, however, less relevant to the invention andare omitted herein to help clarify the drawing and description.Additionally, those persons skilled in this art recognize that siliconcarbide sublimation systems of the type described herein are availableboth commercially and as constructed in a custom fashion as may benecessary or appropriate. They accordingly can be selected or designedby those of ordinary skill in this art without undue experimentation.

The susceptor 21 is typically surrounded by insulation, several portionsof which are illustrated at 23 in FIG. 8. Although FIG. 8 illustratesthe insulation as being generally consistent in size and placement, itwill be understood and is recognized by those of skill in the art thatthe placement and amount of the insulation 23 can be used to providedesired thermal gradients (both axially and radially) along thesusceptor 21. Again, for purposes of simplification, these possiblepermutations are not illustrated herein.

The susceptor 21 includes one or more portions for containing a siliconcarbide powder source 28. Such a powder source is most commonly—althoughnot exclusively—used in seeded sublimation growth techniques for siliconcarbide. FIG. 8 illustrates the powder source as being contained in alower portion of the susceptor 21 and this is one typical arrangement.As another familiar variation, some systems distribute the source powderin a vertical, cylindrical arrangement in which the source powdersurrounds a larger portion of the interior of the susceptor than doesthe arrangement illustrated in FIG. 8. The invention described hereincan be appropriately used in both types of equipment.

The seed crystal of silicon carbide is designated at 24, and istypically placed in upper portions of the susceptor 21. A seed holder 25is typically used to hold the seed 24 in place with the seed holder 25being attached to the susceptor in an appropriate fashion. This caninclude various resting or threaded arrangements. In the orientationillustrated in FIG. 3, the upper portions of the seed holder 25 wouldtypically include threads as would the uppermost portions of thegraphite susceptor 21 so that the seed holder 25 could be threaded intothe top of the susceptor 21 to hold the seed 24 in the desired position.The growing crystal is illustrated by the dotted rectangle designated26.

The general scheme for sublimation growth is set forth briefly in theBackground portion of the specification as well as in a number of thereferences already referred to herein, and in other sources well-knownto those of ordinary skill in this art. Typically, an electric current,having a frequency to which the susceptor 21 responds, is passed throughthe induction coils 22 to heat the graphite susceptor 21. The amount andplacement of the insulation 23 are selected to create a thermal gradientbetween the powder source 28 and the growing crystal 26 when thesusceptor 21 heats the powder 24 to sublimation temperatures, which aretypically on the order of about 2300° C. The thermal gradient isestablished to maintain the temperature of the seed 24 and thereafterthe growing crystal 26 near, but below the sublimation temperature ofsilicon carbide to thereby thermodynamically encourage the vaporizedspecies that are generated when silicon carbide sublimes (Si, Si2C, andSiC2) to condense first upon the seed crystal and thereafter upon thegrowing crystal. As one example, U.S. Pat. No. 4,866,005 suggestsmaintaining the seed at about 2300° C.

For purposes of clarity, the singular term, “thermal gradient,” will beused herein, but it will be understood by those of skill in this artthat several gradients can desirably co-exist in the susceptor 21 andcan be subcategorized as axial and radial gradients, or as a pluralityof isotherms.

If the temperature gradients and other conditions (pressure, carriergases, etc.) are properly maintained, the overall thermodynamics willencourage the vaporized species to condense first on the seed crystal 24and then on the growing crystal 26 in the same polytype as the seedcrystal 24.

FIG. 12 is an exaggerated schematic illustration of some of the factorsrelated to the present invention. It will be understood that FIG. 7 isentirely schematic and illustrative in nature and is not meant torepresent, either in scale or in exact shape, any particular crystal.

FIG. 12 a illustrates a crystal seed broadly illustrated at 30 withthree different types of growth surfaces being respectively indicated at31, 32 and 33. The growth surface indicated by the arrow 31 is concaveand according to the invention will be considered to be excessivelyconcave if the growth surface deviates from flat by more than 10 μm.

The surface illustrated by the arrow 33 is considered excessivelyconvex, again because the deviation from a flat surface is more than 10μm.

An appropriate flat surface is indicated by the arrow 32 and extendsacross the entire usable area of the crystal, the boundary of which isdesignated by the arrow 34.

The FIG. 12 b schematically illustrates problems that can arise in“local” areas of the crystal surface in which the growth surface isindicated at 36 and the dimensions of the local area (as defined herein)are indicated by the horizontal arrow 37. The arrows 40 and 41 indicaterespective areas of high local curvature that can affect the growth in adisadvantageous manner in the same way as the overall undesirable growthshapes illustrated in FIG. 12 a.

In describing the invention, it will be understood that a number oftechniques are disclosed. Each of these has individual benefit, and eachcan also be used in conjunction with one or more, or in some cases all,of the other disclosed techniques. Accordingly, for the sake of clarity,this description will refrain from repeating every possible combinationof the individual steps in an unnecessary fashion. Nevertheless, thespecification and claims should be read with the understanding that suchcombinations are entirely within the scope of the invention and theclaims.

In considering the proportional dimensions of the diameter and thicknessof the seed crystal, whether expressed as a percentage, a fraction, or aratio, it will be understood that in the context of the improvementsprovided by the invention, these proportions have their inventivemeaning in the context of the larger-diameter seed crystals that aredescribed herein.

Accordingly, in certain embodiments the invention is described andclaimed herein in the relevant embodiments in a manner that includes theabsolute dimensions of the crystal, usually in terms of a diameter, ofwhich 2 inch, 3 inch and 100 mm diameter single crystals are preferred.

The invention provides an improvement in the nature and handling of seedcrystals in the growth environment in a manner that enables largecrystals of desired diameters to be obtained.

In another sense, the nature of certain power devices is such that evenin the semiconductor context, in which devices are typically physicallymuch smaller than are equivalent conventional devices, some devices muststill be relatively large and thus require large substrates and largedefect-free single crystals. The invention provides its advantages inthe context of the larger crystals that are required for such largerdevices. Thus, previous single crystals, even those of high quality,that cannot match the absolute dimensions of the crystals described andclaimed herein are not relevant to the evaluation of the invention.

Even in situations or devices that do not require larger substrates, thenormal and relevant goals of increasing the usable amount of any givenbulk single crystal still apply. Thus the invention offers itsadvantages in terms of such larger bulk crystals rather than in a mereconceptual context in which size has no performance or commercial value.

Although it may initially appear contradictory to refer to seed crystalshaving otherwise thin profiles as being “thick,” it will be understoodthat the goal of the invention is to provide a seed crystal that is thinfor overall efficiency purposes but thick enough for the physical andthermal purposes of the invention. Stated differently, if all otherconditions were perfect or ideal, the amount of seed crystal requiredwould be no more than the smallest that could still encourage singlepolytype growth upon its surface. From the opposite standpoint, ifsilicon carbide were an easy or inexpensive material to grow or produce,the thickness of the seed crystal would not matter for efficiencypurposes. In the present invention, however, the goal is to maximize theuse of each seed crystal, which encourages the use of thin seeds. Withinthe context of efficiently thin seeds, however, the ratios described inclaim herein helps solve the described problems.

FIGS. 9 and 10 are schematic illustrations of the relative sizedifferential in the seed crystals according to the present invention. Inboth figures the seed holder is illustrated at 25, the seed crystal at24 and the growing seed at 26. Thus, FIG. 9 illustrates (in admittedlyexaggerated fashion) a more conventional “thin” seed crystal, while FIG.10 illustrates a proportionally thicker crystal according to theinvention.

In another embodiment, the invention is the improvement in seededsublimation growth of silicon carbide that comprises lapping both sidesof a silicon carbide seed crystal, to minimize or eliminate any warp orbow in the crystal that would otherwise encourage sublimation from therear of the seed crystal or undesired thermal differences across theseed, thereafter initiating seeded sublimation growth using the lappedcrystal, and thereafter growing the bulk single crystal at substantiallythe same diameter as the seed. In preferred embodiments, the seed is atleast 2 inches in diameter and most preferably larger, includingdiameters of 3 inches and 100 mm.

In the first embodiment, the purpose of obtaining the proportionallythicker crystal is to prevent the high temperatures to which the crystalis typically exposed from causing the crystal to warp or bow under thethermal stresses generated by those high temperatures. In the nextembodiment, the goal is similar; i.e., to prevent undesired crystalgrowth effects from taking place at the seed crystal, particularly at orfrom the rear of the seed crystal. In this embodiment, however, lappingand polishing the seed crystal provides an extremely smooth and levelsurface that helps discourage localized temperature or physicaldifferences across the seed during growth and that also helps tominimize or eliminate the space or spaces that have been identified asbeing present between the seed crystal and the seed holder in someembodiments.

In a typical technique the crystal is lapped by using an abrasive slurryin conjunction with a hard surface to quickly remove a fairly largeamount of material, and thereafter polished by using a similar slurry,but in a milder fashion and with a softer surface to produce a finishedsurface of semiconductor material. Such techniques are well known in theart and can be selected and used by those of ordinary skill withoutundue experimentation and thus will not be described in further detailherein.

In another embodiment, the invention is the improvement in seededsublimation growth that comprises positioning the seed crystal in acrucible while minimizing or eliminating torsional forces on the seedcrystal to thereby prevent the torsional forces from warping or bowingthe crystal in a manner that would otherwise encourage sublimation fromthe rear of the seed crystal or undesired thermal differences across theseed.

Once again, the goal is to use the nature and positioning of the seedcrystal in a manner that prevents undesired defect generation in theseed during sublimation growth to thereby prevent those defects frombeing propagated into the growing bulk crystal.

In this embodiment, the method preferably comprises positioning the seedcrystal on a seed holder and placing the seed holder and the seedcrystal in the crucible portion of the seeded sublimation system. Mostpreferably, this comprises mounting the seed crystal in an edge ringseed cap and positioning the edge ring seed cap in the seed holder tothereby minimize the mechanical forces applied to the seed crystal to inturn minimize or eliminate any distortion in the seed crystal resultingfrom its mounting on the seed holder.

Although the down-force applied to the ring has not been directlymeasured, by inference from other measurements and techniques, the forceis estimated to be about one Newton and the dimensions of the ring aresuch as to apply this amount of force to the outer 1 mm of the seedwafer.

FIG. 11 illustrates an edge ring seed cap of this type designated at 27,with the other common elements carrying the same reference numerals asin FIGS. 9 and 10. Thus, the edge ring seed cap 27 carries the seedcrystal 24 and thereafter the growing crystal 26. In turn, the edge ringseed cap 27 is threaded to the seed holder 25, which is incorporated inthe overall system as schematically illustrated in FIG. 8.

As noted above, this embodiment can also include the advantages of theprevious to embodiments; i.e., the crystal can be “thick” enough forthermal purposes and can be double lapped to provide the desirablysmooth surfaces.

In yet another embodiment, the improvement according to the inventioncomprises annealing the seed holder prior to sublimation growth toprevent the seed holder from significant distortion during crystalgrowth at silicon carbide sublimation temperatures to thereby minimizeor eliminate temperature differences across the seed that wouldotherwise tend to initiate and propagate defects into the growing bulkcrystal. Thereafter, the method comprises initiating seeded sublimationgrowth with the seed crystal, and then growing the bulk single crystalat substantially the same diameter as the seed crystal.

The time and temperature profile for annealing the seed holder can beselected by those of ordinary skill in this art without undueexperimentation based upon the relatively well understoodcharacteristics and properties of materials such as graphite which aretypically used in most suitable susceptors and seed holders. In aparticular embodiment using a graphite seed holder, however, the methodcomprises annealing the seed holder at a temperature of at least about2500° C. for a period of at least about 30 minutes.

The relevant terms will be understood by those of ordinary skill in thisart, but for purposes of confirmation, the term “uniform” refers to aconsistent surface as observed by the eye or with a microscope.“Uniform” also refers to consistency for areas ranging in lateraldimensions from the wafer diameter down to areas as small as 10 μm. Theterm “flat” means that the crystal forms a basal facet in such a region.Flat also refers to a slope of less than one degree and the term“slightly convex” refers to a radius of curvature of less than 40 cm.Thus, the surface is “too convex” if its overall convexity exceeds thestated levels and the surface is “too concave” if the growth surfacedeviates from flat by more then the stated angle (i.e., preferably onedegree).

Those familiar with the growth of silicon carbide crystals will alsounderstand that this undesired curvature can occur in localized areasrather than over the entire seed, but with the same effects that shouldbe avoided. Such localized regions are best understood as being areasthat are small as 10 μm in diameter (or across their largest dimensionif irregular) up to the full wafer diameter which is typically on theorder of 2 in., 3 in., or 100 mm.

In this aspect, the method preferably comprises maintaining the flat toslightly convex growth surface during the first 500 μm to 10 mm of thecrystal growth over the usable diameter of the crystal. As part of thisaspect, it has been discovered that the shape of the growing crystal atthe early stages of growth is more important than has been generallyrecognized to date. Conventionally, the growth is consideredsatisfactory if the overall resulting bulk crystal has a convex surfacewhen growth is complete or otherwise large enough to observe. Incontrast, the present invention includes the recognition that concavegrowth at early stages tends to generate micropipes that propagatethroughout the crystal even if the end result is an appropriatelyconvex-surface bulk crystal. As noted elsewhere herein, conventionalthought believes that micropipes initiate at the rear (non-growth)surface of the seed crystal. In contrast, the invention recognizes thatmicropipes initiate at the front surface (the seed-growth interface) andtheir avoidance must be addressed at that location as well as elsewhere.

The term “usable area” is likewise understood by those of ordinary skillin this art and represents the area of the crystal that will be selectedand used to fabricate substrates. As an example, the crystal could begrown with a diameter of 150 mm for the purpose of cutting a 75 mmdiameter portion and making wafers from this smaller portion. In such acase, the usable area would be represented by the selected 75 mmportion. As another example, a 150 mm crystal could be directly cut into150 mm wafers of which only the center 75 mm diameter portion is highquality material with the outer portion being lower-quality material. Inthis case, the usable area would refer to the high quality (low defectlevel) region.

The preferred control over the early growth thicknesses of between 500μm and 10 mm defines the invention in contrast to previous work in whichboules of silicon carbide have been produced and then either polished,cut, or sometimes grown, with a resulting flat top surface. In contrast,the invention represents the step of starting growth with a flat seed.

Without being held to any particular representations or admissions, theinventors believe that crystals as small as 10 mm in thickness havenever been demonstrated with the shape (flat) characteristics disclosedand claimed herein. With respect to the 500 μm lower limit, thisrepresents a reasonable general border that distinguishes the inventionfrom growth processes in which the growth is carried out in concavefashion for 500 μm and thereafter flattened.

The method can further comprise using the temperature distribution inthe sublimation crucible to produce the uniform flat to uniform slightlyconvex growth face. As is generally well understood in this art, thetemperature profile in a crucible can be controlled using severaldifferent techniques to obtain desired gradients, isotherms, or othertemperature effects. To some extent, these profiles can also be affectedby the position in which the source materials, including silicon andcarbon gas species, are introduced into the crucible.

In certain embodiments, the seed holder is made of graphite and thegraphite is densified by applying organic materials to improve thethermal uniformity of the material under growth conditions. Although theexact density measurements (and their changes) have not been determined,the organic materials that have been found to provide satisfactoryresults include mixtures of alcohols and resins, such as a mixture offurfuryl alcohol and phenolic (“Novalac”) resin in a ratio of 2.5:1. Todate, growth improvements based on densifying the seed holder representempirical observations. Although the inventors do not wish to be boundby any particular theory, possible explanations include theconsideration that porous graphite more readily permits undesiredsublimation from the rear of the seed, or the consideration that porousgraphite exhibits uneven thermal properties.

Phenolic resins are generally well understood in the chemical arts andan appropriate composition can be selected by those of ordinary skill inthis art without undue experimentation. As a brief summary, phenolicresins are the condensation products of phenol or substituted phenolswith aldehydes. Phenolic resins are thermosetting and do not includeelements such as sulphur or nitrogen that tend to generate casting ormolding defects in a finished seed holder and that would generally bedisadvantageous in the sublimation techniques described herein.

In another aspect, the seed holder is graphite that is selectivelymatched in terms of its coefficient of thermal expansion to the siliconcarbide seed material to help ensure that the gap between the seed inthe seed holder does not exceed 10 μm. A number of grades of graphiteare commercially available and an appropriate graphite for this purposecan be selected by those of ordinary skill in this art without undueexperimentation.

In another aspect of the invention, the thermal consistency between theseed crystal and the seed holder is accomplished by separating the seedand the seed holder in an arrangement that encourages radiative heattransfer to dominate over conductive heat transfer over substantiallythe entire usable area of the crystal. This embodiment has the sametheme, goal, and function as the earlier embodiments-avoiding thermalinconsistencies (typically conductive ones) between the seed and theseed holder.

In this aspect, however, the thermal consistency is achieved bydesigning and keeping a sufficient gap between the seed and the seedcrystal so that conductive heat transfer is avoided, thus avoiding thepotential for inconsistent conductive heat transfers to encouragedefects in the seed or in the growing crystal. As used herein, the term“gap” refers to the distance between the seed and the seed holder, butis not necessarily limited to describing empty space.

Thus, the gap can be filled with any desired material—solid, liquid, orgas—or it can even be a vacuum, provided it serves the required purposeand does not otherwise interfere with the sublimation growth.

As noted in the Background, sublimation from the rear of a seed crystal(“thermal etching”) has been observed and can initiate defects.Accordingly, in another aspect, a thin film can be applied to the backsurface of the seed to prevent thermal etching of that surface, or toprevent sublimation growth on the rear surface of the seed that wouldcreate an undesired conductive heat transfer between the seed and theseed holder. The thin film creates a diffusion barrier that prevents thesilicon carbide from subliming through the barrier. The thin film isselected from materials that will remain stable without subliming,melting, or evaporating at the growth temperatures, and that will nototherwise interfere with the sublimation growth techniques. Appropriatematerials include but are not necessarily limited to, graphite, certainrefractory metals, and metal carbides such as titanium carbide.

As in the other embodiments, the silicon carbide seed crystal ispreferably selected to have a polytype selected from the groupconsisting of the 3C, 4H, 6H, and 15R polytypes of silicon carbide.

Additionally, it will be understood that the methods described hereinare complementary rather than exclusive of one another, and thus severalof the steps can be used concurrently or sequentially to achieve thedesired result. Furthermore, the method has similar advantages for othersemiconductor materials such as gallium nitride, aluminum galliumnitride, and aluminum nitride.

In yet another embodiment, the invention comprises the improvement inseeded silicon carbide growth of positioning a seed crystal on a seedholder using a low porosity backing material that provides a vaporbarrier to silicon carbide sublimation from the seed and that minimizesthe difference in thermal conductivity between the seed and the backingmaterial to minimize or eliminate temperature differences across theseed and likewise minimize or eliminate vapor transport from the rear ofthe seed that would otherwise initiate and propagate defects from therear of the seed towards the growth surface. In particular, the backingmaterial should be selected so that its coefficient of thermal expansionat sublimation growth temperatures is as close as possible to that ofthe SiC seed crystal.

As used herein, the “back” or “rear” of the seed crystal refers to thesurface that faces the seed holder. Thus, the “front” of the seedcrystal represents the surface upon which bulk growth takes place.

Again, the technique of the embodiment is consistent with the goal ofpreventing sublimation from the rear of the seed crystal during growthbecause such sublimation tends to generate defects into and through theseed crystal and thereafter into and through the growing bulk singlecrystal.

In another aspect of this embodiment, the seed can be positioned on theseed holder with a high temperature adhesive, one examples of whichincludes graphite glue.

In yet another aspect of this embodiment, the method can comprisebonding the seed crystal to the seed holder with an appropriatematerial.

In the drawings and specification there has been set forth a preferredembodiment of the invention, and although specific terms have beenemployed, they are used in a generic and descriptive sense only and notfor purposes of limitation, the scope of the invention being defined inthe claims.

1. In a method of producing a high quality bulk single crystal ofsilicon carbide in a seeded growth system, the improvement comprising:positioning the seed crystal in a crucible while exerting minimaltorsional forces on the seed crystal to thereby prevent torsional forcesfrom warping or bowing the seed crystal in a manner that would otherwiseencourage sublimation from the rear of the seed crystal or undesiredthermal differences across the seed crystal.
 2. A method according toclaim 1 comprising positioning the seed crystal on a seed holder andplacing the seed holder and seed crystal in the crucible portion of aseeded sublimation system.
 3. A method according to claim 2 comprisingmounting the seed crystal in an edge ring seed cap and positioning theedge ring seed cap on the seed holder to thereby minimize the mechanicalforces applied to the seed crystal and thereby minimize or eliminate anydistortion in the seed crystal resulting from its mounting on the seedholder.
 4. A method according to claim 1 comprising initiating growthwith a seed crystal of silicon carbide that has a polytype selected fromthe group consisting of the 3C, 4H, 6H, and 15R polytypes.
 5. A seededsublimation growth method according to claim
 1. 6. In a seeded growthsystem for producing a high quality bulk single crystal of siliconcarbide, the improvement comprising: a threaded upper portion of theseed holder; and a threaded upper portion of the susceptor; forthreading the seed holder into the top of the susceptor to hold the seedin the desired position.
 7. A sublimation growth system according toclaim
 6. 8. A growth system according to claim 6 further comprising aseed crystal of silicon carbide that has a polytype selected from thegroup consisting of the 3C, 4H, 6H, and 15R polytypes.
 9. In a seededgrowth system for producing a high quality bulk single crystal ofsilicon carbide, the improvement comprising mounting the seed crystal inan edge ring seed cap and positioning the edge ring seed cap in the seedholder to thereby minimize the mechanical forces applied to the seedcrystal to in turn minimize or eliminate any distortion in the seedcrystal resulting from its mounting on the seed holder.
 10. A growthsystem according to claim 9 further comprising a seed crystal of siliconcarbide that has a polytype selected from the group consisting of the3C, 4H, 6H, and 15R polytypes.
 11. A sublimation growth system accordingto claim 9.